Some things, conflicts, insecurities etc can be conveyed with very little and it doesn't even have to be spoken out loud. You don't have to use up 10-15 minutes of a film to explain all that is going on under the surface with Jane, Thor and Sif. Even in Thor 1 there are hints at Sif's feelings towards Thor. They obviously have a past, and we'll see more of what past they have in the coming film. Jane will probably pick up on this and all sorts of insecurities might happen. Or not! But those are things that can play out very much in silence, while other things are happening, with a line or two at times to underline what's going on.